1. Field of the Invention
The present invention concerns automatic control of artificial lighting to obtain a satisfactory brightness.
The problem is to slave the lighting load providing the artificial lighting to the actual brightness in a given area, for example a user's worksurface.
At present, the lighting load is usually continuously controlled for this purpose.
The present invention is directed to the situation in which, in contrast, for diverse reasons including economic ones, the lighting load is controlled on an on/off basis by delivering to it alternately forced turning on instructions and forced turning off instructions.
2. Description of the Prior Art
This is the situation, for example, when the lighting load employs fluorescent lamps, because continuous control of these lamps requires an electronic ballast, which is particularly costly, and so continuous control of these lamps is rarely used in practice, in some geographical areas at least, and depending on installation practice, in particular because of its cost.
When the lighting load is controlled on an on/off basis, the brightness measured in the given area concerned is usually compared with particular brightness thresholds.
In one prior art implementation, for example, a forced turning on instruction is systematically delivered if the brightness measured is less than a given percentage of that which the lighting load can provide and a forced turning off instruction is systematically delivered if the measured brightness is above a different given percentage, in practice a higher given percentage, of that brightness, in practice with allowance systematically being made for the brightness difference between the brightness values measured before and after a forced turning off instruction, for example using an average calculated over several turning off instructions. This is for "training" the system.
In one such prior art implementation the lighting load is therefore controlled independently of the user.
Also, inevitable aging of the lighting load concerned is not taken into account.
What is more, an implementation of the above kind cannot take account of variations in brightness due to failure of a lamp. The brightness produced by the lamps may then be very different from that which can be assured by the lighting load and which provides the basis for defining the percentages used for the forced turning on and off instructions. In this case, the forced turning on and off instructions may correspond to brightness thresholds that are uncomfortable for the user.
A general aim of the present invention is to provide a method and apparatus allowing for the requirements of the user in order to improve user comfort.